On local Area Networks (LANs) of the Ethernet type able to transmit data at speeds higher than one gigabit/s, an unlimited number of stations can be connected to a shared medium. To control the exchange of data between the stations connected on the shared medium, Ethernet uses a protocol called Carrier Sense, Multiple Access Collision Detect (CSMA/CD). The “Multiple Access” part means that every station is indeed connected to the shared medium forming a single data path. The “Carrier Sense” part means that before transmitting data, a station checks to see whether any other station is already transmitting. If the transmission medium appears to be idle, the station may begin to send data. However, two stations can start transmitting at the same time, causing a collision. When this occurs, each interfering station detects the collision. Hence, all stations attempt to transmit, back off, and try retransmissions at randomly selected later times, thus minimizing the chance of another collision.
Although Ethernet does not set an upper limit to the number of stations that can be connected on the same transmission medium, there are, in practice, drastic limitations. Generally speaking, as more users are added to a shared network, or as applications requiring more data are added, performance inevitably deteriorates. This is because the users become competitors in trying to use a common resource: the shared transmission medium. It is generally agreed that a moderately loaded 10 Mbps Ethernet shared by 30–50 users can sustain a throughput in the neighborhood of only 2.5 Mbps after accounting for packet overhead, inter-packet gaps, and collisions resulting in the use of the CSMA/CD protocol. Thus, although simple, CSMA/CD protocol is limited in its ability to take advantage of the intrinsic performance of the shared transmission medium i.e., 10 Mbps in this example.
Further increasing the number of users (and therefore packet transmissions) creates an even higher collision potential. Since collisions occur when two or more stations attempt to send information at the same time, when these stations realize that a collisions has occurred, they must, to obey the Ethernet standard, all shut off for a random time before attempting another transmission. This tends to add a considerable overhead, severely impacting performance. Consequently, the Ethernet mechanism collapses when the shared medium is overloaded.
One well-known solution to this problem is to segment traffic over independent, disjoint, smaller collision domains, which comes at the expense of having to put in place extra devices to allow communication between the independent pieces of the LAN thus created. This may be accomplished by using a bridge or a switch. For example, an eight-port high-speed switch can support eight Ethernets, each running at a full 10 Mbps and thereby interconnects more users on what appears to them as a single LAN. Such a solution, which creates a more expensive and complicated network, goes against the original objectives of the Ethernet LAN, which were to provide a very inexpensive solution, simple to manage for local communications over a campus or between the employees of a company dispersed over a group of buildings.
Another solution consists in implementing a token-passing mechanism of Token Ring LAN so that the physical Ethernet network becomes collision free and therefore can be used at higher rates. In this mechanism, a logical ring is formed between connected stations and a token is circulated among the connected stations that are part of the logical ring. Then, a station of the logical ring is permitted to transmit only while holding the token, thereby preventing collisions from happening.
Although the above system prevents collisions between stations, it does not prevent a station from monopolizing the bandwidth and preventing other stations from transmitting.